The objective of a planning process, in marketing or elsewhere, is typically to pick the most worthwhile goals and figure out how to get there in the most efficient way possible.  Usually,  the most significant barrier to effective  planning for many is  oversimplification and basing decisions on assumptions and "feel-good" information that has no or little relation to reality. This leads to really bad decisions and I believe is a primary reason that statistically, most  businesses and plans fail.